This invention concerns gates or doors which, when retracted, are essentially side by side and which, when extended, are almost end to end, with a small overlap, so that a wide opening can be closed with two or three gates shorter than the width of the opening. Such gates are commonly referred to as stacking gates. Means are provided for extending and retracting such gates from one side of the opening.
It is not uncommon to close driveways with a gate that rolls to one side of the driveway along a track for opening the driveway. In some situations, to close a driveway 20 feet wide, a gate as much as 24 feet long may be used, the extra length being used to keep the gate vertical where there is no overhead guide track for the gate. In such a situation, a pocket or other location beside the driveway must be provided for the gate when it is retracted. Providing such a space for the retracted gate can sometimes be a problem. It is desirable to have gates that stack into a pocket narrower than the width of the opening that is being closed.
As openings become wider, the problems become greater. It would be desirable, for example, to have two gates substantially less than 40 feet long to close a 40-foot opening. Such stacking gates are readily provided in structures where there is overhead support since mechanisms for extending and retracting the gates can be provided overhead. This can be used in a building entrance, for example. A variety of arrangements have been provided for multiple door closures of large openings such as may be found in airplane hangers.
A problem with open-air gates on the other hand is the lack of overhead structure on which extending and retracting mechanisms can be mounted. An alternative would be to mount such an extension and retraction mechanism in a trench beneath or beside the track. This is extremely troublesome in inclement weather, particularly when there are freezing cycles. Such subterranean mechanisms are also subject to corrosion problems and maintenance difficulties.
Various extension and retraction mechanisms employing flexible cables have been devised for doors. Cables, however, have a relatively limited lifetime because of the flexing as the cables run over the various pulleys in such a system. Some gates may open and close a few hundred times per day, and the cable systems are subject to rapid deterioration and extensive maintenance requirements in such situations. It would, therefore, be desirable to employ a roller chain drive instead of a cable system, since the roller chain suffers very little from repeated flexing.
A roller chain, however, is flexible only in one dimension. It can flex around the direction of the axes of the rollers, but cannot bend to the side to any significant extent. Thus, any system employing roller chain must have the drive system in substantially a single plane. Drive systems suitable for use of cables cannot necessarily be adapted to use of roller chain, since they often do not remain in a single plane.
An arrangement sometimes used for stacking doors drives the leading door. When it has travelled a major part of its length, a dog engages the second door and drags it along. When the second door has travelled a major part of its length, a dog on the second door picks up a third door and drags it along. And so on. This closes an opening at the rate one drives the leading door. One problem with such an arrangement is that the load on the driving mechanism continually increases as additional gates are picked up. It must be designed for the load of all the gates and when pulling only one gate it may be significantly overpowered and damage something in event of a gate jamming or the like.
Thus, it is desirable that the extending and retracting forces be approximately constant throughout the travel of the gates. It may also be desirable to close the opening faster than the drive, by driving the trailing door and amplifying the motion of the trailing door to drive the leading door twice as fast.